German Patent Application No. 43 29 919 describes an output stage for operating a load arranged in a motor vehicle, which contains two series circuits of a first transistor with a second transistor, connected between a first and a second power supply terminal. The four output stage transistors form a bridge circuit, in whose diagonals the electrical load is arranged. The current flowing through each of the four output stage transistors is detected by a current sensor for each and compared with a predefined maximum value in a comparator. If a threshold is exceeded, the respective output stage transistor is turned off. Using inexpensive current sensors, which do not allow electrical isolation, the conventional circuit needs four comparators, each at a different potential.
An object of the present invention is to provide a circuit for detecting an impermissibly high current in an output stage, using the least possible number of simple electronic components.
The present invention provides a series circuit of a first transistor with a second transistor, connected between a first and a second power supply terminal, an input of a comparator being connected at the connection point of the two transistors. The other input of the comparator is connected at the center tap of a voltage divider connected between the two power supply lines. Change-over switching means, which are controlled by the switching signal of one of the two transistors, for changing the voltage divider ratio are provided.
The circuit according to the present invention detects the voltage drop across the gap of the transistors when the gap is closed. The two transistors are arranged, for example, in a half-bridge or full-bridge circuit. The two transistors are alternately switched on or off. An input signal of one transistor can therefore be used to actuate the change-over switching means which change the voltage divider ratio. The switching threshold of the comparator can be adapted by this change either to the first transistor or to the second transistor. In normal operation, the output signal of the comparator changes to a different state when the input signal of one of the two transistors changes. Based on this output signal of the comparator, a change in the other signal state also takes place in the event of an overcurrent; this change is detected in an additional circuit (not illustrated) and can be used, for example, to turn off one transistor or all transistors.
The essential advantage of the circuit according to the present invention is that no additional current sensor and only one comparator is needed for the two transistors. The electronic components used are relatively simple and therefore inexpensive.
The circuit according to the present invention is well-suited in particular for use in a half-bridge circuit in which the other bridge branch is formed by two capacitors. In particular, the circuit according to the present invention is well-suited for use in a full-bridge circuit in which the capacitors of the half-bridge circuit are replaced by output stage transistors. The circuit according to the present invention can also be used in this additional bridge branch, so that a total of two comparators and two change-over switching means are required for a full-bridge circuit.
The output signal of the comparator which in normal operation changes as a function of the input signals is advantageously sent to a logic selector circuit, which is responsible for a signal change at the output of the logic selector circuit as a function of the input signal of one of the two transistors only if an impermissibly high current is detected in one of the two transistors. A particularly simple implementation of the logic selector circuit is possible through an exclusive-or (EXOR) arrangement.
Another advantageous embodiment concerns the change-over switching means, which preferably contain a switching transistor, which connects a shunt resistor to a resistor of the voltage divider.